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Title: Subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab/mantle interface

Abstract

Seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested. These studies, employing waves reflected and converted at the slab/mantle interface, suggest that a velocity contrast of 5-10% occurs over a distance of 10-20 km to depths of at least 350 km. The authors have investigated whether such sharp contrasts can result form thermal structure, compositional differences, phase changes, or other effects such as preferred mineral orientation. Using a mantle and slab temperature model, a petrologic model of mantle and slab mineralogy, and a data base of elastic properties of mantle and slab phrases, theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints. At 9.6 GPa and 1000C (conditions modeled at the slab/mantle interface at {approximately}280-km depth), cooler temperatures in the slab interior produce, at constant composition, only approximately a 1.75% V{sub p} increase into the slab in any of the peridotitic and eclogitic mineralogies investigated. Variation in V{sub p} among the different bulk compositions is only approximately 0.5%. These contrasts are substantially lower than those obtained from the seismological studies andmore » indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts. They propose that phase transformations in mantle and slab mineralogies, notably elevation of the olivine-spinel phase change normally occurring at 400-km depth, are important factors in these sharp contrasts. Other contributing effects may include preferential mineral orientation near the slab surface and, at shallower depths, the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface.« less

Authors:
; ;  [1]
  1. Northwestern Univ., Evanston, IL (USA)
Publication Date:
OSTI Identifier:
5270234
Resource Type:
Journal Article
Journal Name:
Journal of Geophysical Research; (United States)
Additional Journal Information:
Journal Volume: 94:B1; Journal ID: ISSN 0148-0227
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; SUBDUCTION ZONES; MINERALOGY; SEISMIC SURVEYS; TEMPERATURE GRADIENTS; BOUNDARY CONDITIONS; CONTINENTAL CRUST; DATA ANALYSIS; DEPTH; EARTH MANTLE; ELASTICITY; GEOLOGIC MODELS; INTERFACES; OLIVINE; ORIENTATION; PETROLOGY; PHASE TRANSFORMATIONS; PLATE TECTONICS; ROCK MECHANICS; SEISMIC WAVES; SPINELS; THICKNESS; ALKALINE EARTH METAL COMPOUNDS; ALUMINIUM COMPOUNDS; ALUMINIUM OXIDES; CHALCOGENIDES; DIMENSIONS; EARTH CRUST; GEOLOGY; GEOPHYSICAL SURVEYS; IRON COMPOUNDS; IRON SILICATES; MAGNESIUM COMPOUNDS; MAGNESIUM OXIDES; MAGNESIUM SILICATES; MECHANICAL PROPERTIES; MECHANICS; MINERALS; OXIDE MINERALS; OXIDES; OXYGEN COMPOUNDS; SILICATE MINERALS; SILICATES; SILICON COMPOUNDS; SURVEYS; TECTONICS; TENSILE PROPERTIES; TRANSITION ELEMENT COMPOUNDS; 580000* - Geosciences

Citation Formats

Helffrich, G R, Stein, S, and Wood, B J. Subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab/mantle interface. United States: N. p., 1989. Web. doi:10.1029/JB094iB01p00753.
Helffrich, G R, Stein, S, & Wood, B J. Subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab/mantle interface. United States. doi:10.1029/JB094iB01p00753.
Helffrich, G R, Stein, S, and Wood, B J. Tue . "Subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab/mantle interface". United States. doi:10.1029/JB094iB01p00753.
@article{osti_5270234,
title = {Subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab/mantle interface},
author = {Helffrich, G R and Stein, S and Wood, B J},
abstractNote = {Seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested. These studies, employing waves reflected and converted at the slab/mantle interface, suggest that a velocity contrast of 5-10% occurs over a distance of 10-20 km to depths of at least 350 km. The authors have investigated whether such sharp contrasts can result form thermal structure, compositional differences, phase changes, or other effects such as preferred mineral orientation. Using a mantle and slab temperature model, a petrologic model of mantle and slab mineralogy, and a data base of elastic properties of mantle and slab phrases, theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints. At 9.6 GPa and 1000C (conditions modeled at the slab/mantle interface at {approximately}280-km depth), cooler temperatures in the slab interior produce, at constant composition, only approximately a 1.75% V{sub p} increase into the slab in any of the peridotitic and eclogitic mineralogies investigated. Variation in V{sub p} among the different bulk compositions is only approximately 0.5%. These contrasts are substantially lower than those obtained from the seismological studies and indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts. They propose that phase transformations in mantle and slab mineralogies, notably elevation of the olivine-spinel phase change normally occurring at 400-km depth, are important factors in these sharp contrasts. Other contributing effects may include preferential mineral orientation near the slab surface and, at shallower depths, the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface.},
doi = {10.1029/JB094iB01p00753},
journal = {Journal of Geophysical Research; (United States)},
issn = {0148-0227},
number = ,
volume = 94:B1,
place = {United States},
year = {1989},
month = {1}
}